Titanium dioxide (titania) is extensively used as pigments, catalysts, inorganic membranes, semi-conductors, optical coating reagent and as photocatalysts in water purification process. Titanium dioxide (TiO2) has two phases of crystalline structure of industrial importance, namely, anatase and rutile. Titanium dioxide with anatase phase has been used as a photocatalyst for photodecomposition of acetone, phenol or trichloro ethylene, oxidation such as nitrogen mono-oxide and nitrogen dioxide and conversion system using solar energy due to its high photo-activity. Titanium dioxide with rutile phase has been widely used as white pigment because of its good scattering effect that protects the ultraviolet light. It has also been used in optical coatings, beam splitter and anti-reflection coatings since it has a high dielectric constant and high refractive index, good oil absorption ability, tinting power and chemical stability even under strongly acidic or basic conditions. Titanium dioxide shows different electrical characteristics according to oxygen partial pressure since it has wide chemical stability and non-stoichiometric phase region. Because of this, it can also be used as a humidity sensor and as high-temperature oxygen sensor and the field of its use has become wide.
Titanium dioxide powders for pigment use generally have an average particle size of 150 to 250 nanometer and is considered the principal white pigment of commerce. It has an exceptionally high refractive index, negligible color and is quite inert. Titanium dioxide having a smaller average particle size, for instance in the 10 to 100 nanometer median particle size range, is used commercially in cosmetics and personal care products, plastics, surface coating, self—cleaning surfaces, and photo voltaic applications. This grade of titanium dioxide is referred to as ultrafine or nano-sized titanium dioxide. More than four million tons of titanium dioxide are produced annually; there are several processes for making ultrafine titanium dioxide, some in commercial use and some in development. Some use anhydrous titanium dioxide. Some use anhydrous titanium tetrachloride as a feed stock. Another process uses a titanyl sulfate solution as the feed stock.
Generally, titanium dioxide powders are manufactured by a chloride process, which is a gas phase process, or by a sulfate process, which is a liquid phase process.
In the chloride process, which was commercialized by Du Pont of USA in 1956, titanium tetrachloride is used as a starting material and the reaction temperature needs to be higher than 1,000° C. This method also requires extra protection devices because of the corrosive Cl2 gas product at high temperature in the process, leading to higher production costs. Because titanium dioxide powders produced by the chloride process are fine but rough, additive equipment for providing external electrical fields or controlling reactant mixing ratios are required to control the particle shape and size of titanium dioxide powders. High pure oxygen is required for oxidation of TiCl4 and that leads to high capital and operating costs.
In the sulfate process, which was commercialized by Titan company of Norway in 1961, titanium sulfate (TiSO4) is conventionally hydrolyzed at temperatures higher than 100° C., calcined at 800-1000° C. and then pulverized to produce titanium dioxide powders. During the calcination and pulverization processes, impurities are introduced causing the quality of the final titanium dioxide powder to be low.
Funaki, Saeki, et al. in Kogyo Kagaku Zasshi, 59 (11), pp. 1291 (1956), teach that fine particles of anatase-type titanium dioxide can be produced by mixing titanium tetrachloride and water in the vapor phase, at a temperature in the range from 200° C. to 800° C. or fine particles of anatase-type titanium dioxide containing or not containing a very small amount of rutile-type particles can be produced by the reaction of titanium tetrachloride and water in the liquid phase and a much higher temperature treatment to obtain rutile phase titanium dioxide.
A method for preparing spherical particles of a metal oxide comprising hydrolysis of a hydrolysable titanium (IV) compound in the form of a liquid aerosol by being contacted with water vapor in a dynamic flow is taught in U.S. Pat. No. 4,241,042. A method in which a precursor of a metal oxide in the form of a very fine droplet suspension of the liquid is heated and gasified by evaporation and thermal decomposition and then contacted and reacted with an oxygen containing gas in the vapor phase to give spherical fine particles of a metal oxide is taught in Japanese Patent Kokai 59-107904 and 59-107905.
Recently considerable interest has been directed toward the synthesis of rutile grade titania at low temperature. There have been some reports about new liquid phase processes to synthesize rutile grade titanium dioxide powder using titanium tetrachloride. Kim, Park et al, (U.S. Pat. No. 6,001,326) show a new liquid phase process in which TiO2 precipitates with pure rutile phase having spherical shapes having 200-400 nm in diameter formed between room temperature and 65° C., by the homogeneous precipitation method simply by heating and stirring an aqueous TiOCl2 solution.
Tang et al., (Mater. Chem. Phys. 77 (2): pp. 314, (2003) disclose the preparation of nano-sized rutile TiO2 powder by hydrolysis of Ti (OC4H9) 4 solution at 40-50° C. When the solution is neutral and basic, the hydrolysis product is a precipitate and the dried precipitate is amorphous. The rutile phase TiO2 cannot be obtained even when the dried precipitate is calcined at 600° C. However, when the solution is acidic, the hydrolysis product is a solid, in order to obtain rutile TiO2 by drying the gel at 40-50° C. However, tight control of reaction conditions is required, since alkoxide is intensely hydrolyzed in air. Furthermore, the high price of the alkoxide limits its commercialization.
Yang et al., (Mater. Chem. Phys., 77 (2): pp. 501, (2003) also reported that titania nanocrystals in the rutile form were prepared in liquid phase at room temperature under normal pressure. Li Y. Z. et al, also reported in Jour. Mater. Chem., 12 (5): pp. 1387, (2002), the preparation of nano-crystalline rutile TiO2 with average crystal sizes of 6.9-10.5 nm by hydrolysis of TiCl4 aqueous solution at lower temperatures. All the above reported techniques of synthesizing rutile phase titanium dioxide are based on liquid phase processing.
In comparison with liquid phase routes, vapor phase hydrolysis of titanium chloride to synthesize anatase has been reported. For example; B. Xia et al. (Jour. Mater. Sci., 34, pp. 3505, (1999), reported the preparation of anatase TiO2 nanopowders by vapor-phase hydrolysis of TiCl4 below 600° C. As an independent preparation route, it hasn't been paid much attention.
Compared with liquid phase process, the vapor phase process carried out in an aerosol reactor offers many advantages including product purity, ease of collection, energy efficiency and avoids treatment like filtration, washing, drying etc., involving large liquid volumes. However, the chloride process is carried out at high temperature and has encountered several problems, such as control of product characteristics, reactor corrosion, and operational problems, mainly due to the high temperatures and corrosive gases involved. Therefore, there is a need for a process to produce ultrafine titanium dioxide at temperature much reduced from those encountered in the chloride process but involving only gas phase processing without involvement of liquids.